Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. In addition, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
These implantable neurostimulation systems typically include one or more electrode carrying neurostimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the neurostimulation lead(s) or indirectly to the neurostimulation lead(s) via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the neurostimulation leads to stimulate the tissue and provide the desired efficacious therapy to the patient. The neurostimulation system may further comprise a handheld patient programmer in the form of a remote control (RC) to instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. A clinician, for example, may program the RC by using a computerized programming system referred to as a clinician's programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
In the context of an SCS procedure, one or more neurostimulation leads are introduced through the patient's back into the epidural space, such that the electrodes carried by the leads are arranged in a desired pattern and spacing to create an electrode array. Multi-lead configurations have been increasingly used in electrical stimulation applications (e.g., neurostimulation, cardiac resynchronization therapy, etc.). In the neurostimulation application of SCS, the use of multiple leads increases the stimulation area and penetration depth (therefore coverage), as well as enables more combinations of anodic and cathodic electrodes for stimulation, such as transverse multipolar (bipolar, tripolar, or quadra-polar) stimulation, in addition to any longitudinal single lead configuration. After proper placement of the neurostimulation leads at the target area of the spinal cord, the leads are anchored in place at an exit site to prevent movement of the neurostimulation leads. To facilitate the location of the neurostimulator away from the exit point of the neurostimulation leads, lead extensions are sometimes used.
The neurostimulation leads, or the lead extensions, are then connected to the IPG, which can then be operated to generate electrical pulses that are delivered, through the electrodes, to the targeted spinal cord tissue. The stimulation creates the sensation known as paresthesia, which can be characterized as an alternative sensation that replaces the pain signals sensed by the patient.
The efficacy of SCS is related to the ability to stimulate the spinal cord tissue corresponding to evoked paresthesia in the region of the body where the patient experiences pain. Thus, the working clinical paradigm is that achievement of an effective result from SCS depends on the neurostimulation lead or leads being placed in a location (both longitudinal and lateral) relative to the spinal tissue such that the electrical stimulation will induce paresthesia located in approximately the same place in the patient's body as the pain (i.e., the target of treatment).
To aid the patient care professional in correlating the paresthesia experienced by the patient during stimulation and the dermatomes corresponding the region or regions of pain experienced by the patient, computer programming systems typically include dermatome maps of the human body onto which regions of pain and regions of paresthesia experienced by the patient can be recorded to allow the patient care professional to determine the effectiveness of the therapy. Each dermatome corresponds to a region of the body that is mainly supplied by a single spinal nerve (i.e., a dorsal root (DR) nerve). In each patient, there are eight cervical spinal nerves designated C1-C8, twelve thoracic spinal nerves designated T1-T12, five lumbar spinal nerves designated L1-L5, and five sacral spinal nerves designated S1-S5.
It is believed that large diameter dorsal column (DC) nerve fibers, which extend rostro-caudally along the spinal cord and interact with the DR nerves via the dorsal horn, are the major targets for SCS for overlaying the patient's painful regions with paresthesia. It can then be appreciated that the clinical goal of pain relief can often be achieved by placing the electrodes of the stimulation lead(s) as near as possible to the innervating DC nerve fibers associated with the dermatomic area of pain, and if necessary, “tuning” the electrical stimulation by adjusting one or more stimulation parameters. In some cases, this is relatively simple due to the relatively close proximity of the active stimulating electrodes to the innervating DC nerve fibers, as well as the size and/or orientation of the stimulating electrodes relative to these DC nerve fibers.
However, in some applications of SCS, due to the thickness of the cerebral spinal fluid (CSF) along certain portions of the spinal canal, it is difficult to stimulate DC nerve fibers without also stimulating nearby DR nerve fibers, which may cause discomfort to the patient in the regions in which the DR nerve fibers innervate. This phenomenon can best be appreciated in the context of treating lower back via SCS, where it is very difficult to provide paresthesia to the lower back of a patient without causing uncomfortable chest/abdominal wall sensations due to the stimulation of innervating DR nerve fibers.
For example, with reference to the empirical evidence illustrated in the graphs of FIG. 1, although the maximum probability of achieving paresthesia in the lower back of a patient (approximately 40%) occurs when the T5 spinal level is stimulated as shown in chart 1, the probability of creating side-effects in the form of stimulation of the abdomen (approximately 80%) also occurs when the T5 spinal level is stimulated as shown in chart 2. This phenomenon is mainly due to the fact that as the cerebrospinal fluid (CSF) layer becomes thicker, it becomes more difficult to stimulate DC nerve fibers without also stimulating DR nerve fibers. As shown in chart 3, the maximum thickness of the CSF layer occurs at the T5 spinal level, thereby causing the maximum probability of uncomfortable abdominal stimulation to track the maximum probability of achieving lower back paresthesia.
As a result, clinicians have traded off ineffective stimulation for patient comfort by stimulating the DC nerve fibers well outside of the optimum spinal level range of T4-T6 (shown by band 4), and in particular, well above the T6 spinal level where the probability of achieving lower back paresthesia precipitously drops off. For example, the historical spinal level target for achieving lower back paresthesia is in the T9-T10 range (shown by band 5). Uncomfortable stimulation of the abdomen can be minimized by locating the lead or leads along the centerline of the spinal cord in order to preferentially stimulate the DC nerve fibers over the DR nerve fibers. However, in this case, the probability of achieving lower back paresthesia drops down to the 15-20% range, while the probability of causing uncomfortable abdominal stimulation is still in the 40-55% range. More recently, lower back paresthesia with minimal side effects has been achieved in the T7-T8 range using current steering techniques to refine the resulting electrical field (shown by band 6). In this case, the probability achieving lower back paresthesia is in the 20-30% range.
Because it is difficult to achieve lower back paresthesia without uncomfortable abdominal stimulation, treatment of chronic low back pain via SCS is conventionally treated only ancillary to the treatment of some other ailment, such as chronic leg pain. That is, the lead or leads are implanted in the patient for treating a particular ailment, and if the effective lower back paresthesia can be obtained without significant side effects, than the lower back pain is treated along with the particular ailment.
There, thus, remains a need to provide an SCS regimen that provides relief for chronic lower back pain while minimizing the probability of side effects. The art remains unable to apply SCS in the most beneficial location. There remains a need for an SCS technique that would allow positioning of SCS leads in the optimal therapeutic location for lower back DC stimulation without undesirable collateral effects of stimulating DR nerve fibers.